1. Field of the Invention
The present invention relates to a laser irradiation apparatus and a laser process method which are for irradiating a semiconductor film or the like with a laser beam for crystallization, recrystallization, or activation after ion implantation. Moreover, the present invention relates to a laser irradiation apparatus and a laser process method which are for irradiating a poly-crystalline or near-poly-crystalline semiconductor film with a laser beam to improve the crystallinity of the semiconductor film. The above processes are hereinafter referred to as laser annealing. The present invention further relates to a method for manufacturing a semiconductor device by conducting the above laser process. It is to be noted that the semiconductor device herein described indicates all the devices which can function by using a semiconductor characteristic, including an electro-optic device such as a liquid crystal display device or an electroluminescent (EL) display device and an electronic device which includes the electro-optic device as its component.
2. Related Art
In recent years, a technique for forming a TFT (thin film transistor) over a substrate has significantly progressed and application to an active matrix semiconductor display device has been advanced. In particular, since a TFT using a poly-crystalline semiconductor film has higher electric-field effect mobility (also referred to as mobility simply) than a TFT using a conventional amorphous semiconductor film, high-speed operation is possible. Therefore, such a technique has been developed that a pixel, which has been conventionally controlled by a driver circuit provided outside a substrate, is controlled by a driver circuit formed over the same substrate as the pixel.
A substrate used for a semiconductor device is expected to be a glass substrate rather than a single-crystal silicon substrate in terms of cost. A glass substrate is easily deformed due to heat because of low heat resistance; therefore, in the case of forming a poly-silicon TFT over a glass substrate, laser annealing is often employed for crystallizing a semiconductor film in order to prevent the glass substrate from being deformed due to the heat.
Compared with another annealing method using radiant heat or conductive heat, a laser annealing method has advantages that a process time can be drastically shortened and that a semiconductor substrate or a semiconductor film over a substrate can be selectively and locally heated so a thermal damage is hardly given to the substrate.
The laser annealing method described here includes a technique for recrystallizing an amorphous layer formed in a semiconductor substrate or a semiconductor film, a technique for crystallizing an amorphous semiconductor film formed over a substrate, and a technique applied to flattening or modification of a surface of a semiconductor substrate or a semiconductor film.
In the case of conducting laser annealing, the following method is often used for its high mass-productivity and industrial superiority: a laser beam emitted from a high-output pulsed laser such as an excimer laser is shaped by an optical system into a square spot with several cm on a side or a linear spot with 10 cm or more in length on an irradiation surface, and then an irradiation position of the beam spot is moved relative to the irradiation surface (for example, Reference 1: Japanese Patent Application Laid-Open No. H8-195357).
In particular, when a linear beam spot is employed, unlike the case of using a dot-like beam spot which is required to move from front to back and from side to side, the whole surface of an irradiation surface can be irradiated with a laser beam by moving the linear beam spot only in a direction perpendicular to a direction of the line of the linear beam spot (hereinafter, the direction perpendicular to the direction of the line of the linear beam spot referred to as a width direction). Therefore, high mass productivity can be obtained. Here, the linear beam spot is a rectangular beam spot having a high aspect ratio. The beam spot is moved in a direction of its width because this is the most efficient scanning direction. Because of this high mass productivity, a laser beam emitted from a pulsed excimer laser which is shaped into a linear beam spot by an appropriate optical system has been mainly employed in a current laser annealing step.
With the increase in output power of a laser oscillator in recent years, it has become possible to form a longer linear beam spot than before. Accordingly, the size of a substrate used in laser annealing has become larger and larger. This is because throughput is higher and cost is lower when plural semiconductor devices such as panels for liquid crystal display devices or EL display devices are manufactured with one large substrate than when one semiconductor device such as a panel for a liquid crystal display device or an EL display device is manufactured with one substrate. As a large substrate, for example, a substrate with a size of 600 mm×720 mm, a circular substrate of 12 inch (a diameter of approximately 300 mm), or the like can be used. In the future, a substrate with a size of 2,000 mm or more on a side is considered to be used.
However, for example, an optical path length in an optical system for forming a linear beam spot with a length of 300 mm is as long as 5,000 mm. An optical system having such a long optical path length has problems in that optical alignment is very difficult and an apparatus becomes large because of its large footprint.
If the same optical system is designed so as to have a shorter optical path length, the obtained linear beam spot has, for example, a barrel-like shape or a bobbin-like shape. This is because a point of focus is displaced at opposite ends of the linear beam spot in a direction of its line.
Here, a cause of displacing the point of focus is described. A laser beam entering a lens 2001 obliquely has a longer optical path length than a laser beam entering a lens vertically. Moreover, as an incidence angle of the laser beam entering the lens obliquely becomes larger, an optical path difference increases with respect to the laser beam entering the lens vertically. This difference of the optical path length due to an incident position and an incidence angle causes the point of focus to displace, resulting in that an image blurs as going toward an end of the linear beam spot. In other words, a curved field at which an image is formed on a curved surface as shown in FIG. 14 occurs. Homogeneous annealing cannot be conducted even though an irradiation surface 2002 is annealed with such a beam spot.
With the increase in the substrate size, formation of a beam spot with a length of approximately 1,000 mm in a long-side direction is needed urgently. For example, in the case of conducting annealing to a large substrate such as a substrate with a size of 600 mm×720 mm, the whole surface of the large substrate cannot be annealed only by moving a beam spot having a length of 300 mm in a long-side direction once, and the beam spot needs to be moved at least multiple times; therefore, the throughput decreases. As a result, some regions are annealed multiple times while some other regions are not annealed; therefore, homogeneous annealing cannot be conducted. Consequently, an optical system is designed which forms a linear beam spot having a length of, for example, 1,000 mm which can anneal the whole surface of a large substrate only by moving the linear beam spot in one direction. However, a point of focus is displaced on an irradiation surface at opposite ends of the linear beam spot in a long-side direction.
The cause of the displacement of the point of focus at the opposite ends of the linear beam spot in a direction of its line also results from the curved field occurring when the difference in the optical path length due to the incident position and the incidence angle causes the point of focus to displace, similarly to the cause when the optical path length is shortened. Homogeneous annealing is difficult to conduct with such a beam spot even when the length in the long-side direction is made longer to conduct homogeneous annealing.